1. Field of the Invention
The present invention relates to a stationary induction apparatus such as a transformer or a reactor, and more particularly to an insulating structure for a winding thereof.
2. Description of the Background Art
A transformer is a stationary induction apparatus including two windings. The transformer has come into widespread use in power systems for transmission of electric power, and is for transforming a current value and a voltage value through the use of electromagnetic induction between identical frequency circuits. Furthermore, a reactor is composed of one or more windings, and is for adding an inductance to an electric circuit or a power system. In general, among internal cooling modes, there are oil cooling, gas cooling, liquid cooling, and air cooling, while among external cooling modes, there are air cooling, water cooling, and other types of cooling. Additionally, among magnetic circuits, there are an iron core formed by stacking silicon steel plates and an air core having no iron core.
In any case, parts of an electric conductor forming a winding carry different voltages. For insulation between electric conductors, there have been employed insulating methods, such as an oil filled mode using an insulating oil and a fibrous insulating material, a sulfur hexafluoride (SF6) gas filled mode, a resin mold mode, and others. In general, these insulation means comprise a combination of a plurality of materials or layers differing from each other in insulating ability. In one concept, in a conventional transformer or reactor, to enhance the overall insulating ability, an insulating medium such as oil, gas, or air, having a lower insulating ability than a solid, is subdivided by solid insulating layers to shorten the distance in the insulating medium. For example, as mentioned in the xe2x80x9cElectrical Engineering Handbookxe2x80x9d of the Institute of Electrical Engineers of Japan, 1988, pp. 673-674, the conventional transformer is constructed such that each of the oil layers among a high-voltage winding, a low-voltage winding, an iron core and a tank is divided by insulating partitions made from inter-layer insulating paper. It has been known that the insulating ability of this oil layer per unit distance improves by distance-reduction of the width of the oil layer. That is, the shortening of the width distance of the oil layer, divided by the insulating partitions made from the inter-layer insulating paper, not only enhances dielectric strength but also improves the insulating ability between the windings in addition to suppressing discharge phenomena, provided by the insulating portions, which causes dielectric breakdown. Likewise, in a gas insulating device, as is known from Paschen""s law, the gas section is divided for the distance-reduction of the width of each gas layer, thereby boosting the dielectric breakdown electric field.
Furthermore, a problem in insulation arising in the conventional oil filled transformer relates to an electrification phenomenon called xe2x80x9cflow electrificationxe2x80x9d. In general, in an oil filled transformer, an oil is circulated from a lower portion of a winding to an upper portion thereof for cooling. When this oil comes into contact with an insulating material surface, charge transfer takes place in the vicinity of the boundary between the oil and the insulating material. In addition, the charge transferred due to the oil flow is carried away to cause charge separation. Accordingly, the oil and the insulating material are electrified in reverse polarity. If the electrified charge is accumulated on the surface of the insulation material, a strong electric field appears partially in a surface along an oil layer or an insulating member, which results in abnormal insulation. To prevent this flow electrification, there are countermeasures: (1) restraining the oil flow rate to below some limit by utilizing the characteristic in which the charge quantity decreases with a decrease of the oil flow velocity, and (2) adding an additive to the oil to suppress charge-transfer.
Japanese Unexamined Utility Model Application Publication No. (SHO) 58-175618 discloses a technique in which, to achieve the insulation distance reduction, insulating paper is placed to run among cylindrical windings so that the filling is made with the insulating paper. This utilizes the fact that an oil immersed solid insulating layer has a higher insulating ability than an oil layer. However, irregularities on the winding surfaces are unavoidable due to its structure and manufacturing precision, which indicates that difficulty will be encountered in practice in bringing the insulating paper into close contact with the windings. If an oil layer exists on the winding surfaces, an electric field is concentrated in the oil layer due to the difference in dielectric constant between the insulating paper and the oil. Additionally, as mentioned above, since the oil layer is inferior to the insulating paper layer in insulating ability, a poor insulation structure is formed. Accordingly, it is difficult for the insulating paper to display the entirety of its insulating ability.
So far, a method of preventing such a drawback in the insulation structure has been employed for high-voltage rotating machines. For a stator winding of a high-voltage rotating machine above 1 kV, a winding called xe2x80x9cformed-coilxe2x80x9d has been put to use. This has a construction in which a plurality of insulation-coated conductors are bundled and covered with a composite solid insulation of a synthetic resin and mica, and further is inserted into a slot made in an iron core. Gaps develop between these conductors and the solid insulation and between the solid insulation and the iron core due to stress or deterioration occurring, for example, in the manufacturing process, at the start/stop or during the operation, which constitutes a weak point on insulation and causes partial discharge. A way to prevent this is described in xe2x80x9cManufacturing and Maintenance of Electric Coilxe2x80x9d 1990, p.133, translated by Hisayasu Mitsui, et al. published by Kaihatsusha (from the original by H. Seuentz, xe2x80x9cHerstellung der Wicklungen electrischer Maschinenxe2x80x9d). That is, a semi-conductive layer called an internal corona shield is placed inside a solid insulation and between the solid insulation and a conductor so that the same electric potential is maintained with respect to the conductor, while a semi-conductive layer called an external corona shield is placed outside the solid insulation and between the solid insulation and an iron core to maintain the identical electric potential with respect to the iron core. At this time, the semi-conductive layer is constructed to have a high adhesive strength with respect to the solid insulation so that a gap more easily occurs between the semi-conductive layer and the conductor or between the semi-conductive layer and the iron core. Accordingly, even if a gap develops between the semi-conductive layer and the conductor or between the semi-conductive layer and the iron core, the electric field in the interior of the gap is relieved, thereby suppressing the occurrence of partial discharge and preventing the occurrence of weak points in the insulation structure.
If an insulating structure, in which both surfaces of a solid insulation extending perpendicularly to the electric field applying direction are covered with a semi-conductive material as mentioned above, is applied to a transformer or a reactor, then the lowering of the insulating ability may be prevented and the reliability may be improved. Such an insulating structure for a transformer or a reactor is disclosed, for example, in Japanese Unexamined Patent Application Publication No. (HEI) 10-6350 or in PCT International Publication No. W097/45847. However, a conventional high-voltage rotating machine is constructed such that, in the entire winding, the insulation has the same thickness, and the electrical fields in the insulation differs greatly at a high-voltage section of the winding and at a low-voltage section thereof. In other words, in the low electrical field section of the insulation, a greater volume than is needed is used for insulation, and there is room to increase the space factor of the conductor and the iron core. Additionally, since different materials are combined in the solid insulation of a high-voltage rotating machine, when an internal stress occurs, a gap develops at a weak portion in the boundary face between the materials to accelerate the deterioration, which shortens the insulation life.
A description will be given hereinbelow of the PCT International Publication No. W097/45847. This relates to a transformer or a reactor having a winding formed by winding an insulating conductor, similar to a solid power cable comprising a first semi-conductive layer, a solid insulating layer and a second semi-conductive layer arranged coaxially, in that order, from the inside. This PCT International Publication describes that the portion between the conductors is insulated with a solid insulation surrounded by the semi-conductive layers so that the entire voltage is maintainable with the solid insulating layer having a high insulation reliability. However, in the case of this conventional art, since the space factor is reduced for the following two reasons, the dimension increases. One reason is to use a uniform and thick insulating layer, necessary for withstanding the entire voltage during application of a surge voltage, in a portion that requires a different insulating performance at a normal voltage application to between the windings or between the winding and the iron core. The second is because the conductor used has a circular cross section so that a gap develops between the windings on the winding outer side. Additionally, in general, in the case of a solid insulation such as in this conventional art, defects tend to occur in the process of insulation formation in the interior of a solid insulating layer. Even if such defects in the solid insulation are microscopic so that the detection thereof by a test becomes difficult, during the apparatus life over several decades, the defect portions gradually advance causing a shortening of the apparatus life.
Among the electrical stresses the transformer or the reactor receives, in addition to the normal voltage, there may be a lightning surge voltage due to a lightning stroke and a surge voltage such as an opening/closing surge occurring in a circuit breaker, a disconnector, or the like. As stated in xe2x80x9cTransformer Engineeringxe2x80x9d 1972, translated by Fumio Aoki, published by Corona Co., Ltd. (from the original: xe2x80x9cTransformer Engineeringxe2x80x9d 1951, p.444, by L. F. Blume, et al. published by John Wiley and Sons, Inc.), upon application of a surge voltage, the voltage distribution of the winding differs from the normal voltage distribution, and the voltage sharing increases at the ends of a line constituting a highest voltage portion of the winding and at the ground ends or the series ends forming a lowest voltage portion, so that insulation reinforcement becomes necessary. One way to make the voltage distribution more uniform involves providing an electrostatic plate forming a line-potential electric conductor in a state adjacent to a coil existing at a line end and further placing an end-potential electrostatic plate in a state adjacent to a coil existing at the ground end or series end. Thus, the voltage sharing at the winding end portions is reducible at the application of a surge voltage.
Another way, which is more effective, involves reducing the grounded capacitance to bring the voltage distribution depending mainly on a series capacitance close to a normal voltage distribution depending on the inductance of the winding. Thus, for a coaxially arranged winding arrangement in which a plurality of windings are disposed coaxially, the conventional art has employed a method of placing an electrostatic shield in the exterior of the windings to increase the capacitance of the windings and the line ends for accomplishing a balance to the grounded capacitance or a method of adjusting the conductor disposition order in the winding to increase the series capacitance in the winding. However, these methods can disadvantageously complicate the winding manufacturing. Additionally, there is a problem in that the decrease of the grounded capacitance by the electrostatic shield is not applicable to an alternate disposition winding in which a plurality of windings are wound alternately on an iron core.
As described above, in the case of the conventional stationary induction apparatus, for the reduction of the insulation distance, the oil layer section constituting a weak point is packed closely with insulating paper or the like. However, difficulty is encountered in practice in applying such a structure to all the oil gaps, such as between the winding and the insulating paper and between the insulating paper and the insulating paper. As in the case of a rotating machine, when both surfaces of a solid insulation extending perpendicularly to the electric field applying direction are covered with a semi-conductive material, a high insulation reliability is obtainable, but if this structure is used for a stationary induction apparatus, then the space factor decreases, thus increasing the apparatus volume. Additionally, in the case of the employment of only the solid insulation, defects that can shorten the apparatus life tend to occur in the manufacturing process. Moreover, since the winding grounded capacitance exists and the unequal series capacitance arises, the lack of uniformity of the voltage distribution occurs at the application of a surge voltage and the insulation distance is long. Moreover, the lack of uniformity of the initial voltage distribution creates a problem in that the following voltage oscillations also become intense.
Accordingly, the present invention has been developed in order to overcome the above-mentioned problems, and it is an object of the invention to provide a stationary induction apparatus which has superior insulation performance against a surge voltage and which is capable of removing a weak portion in insulation on a winding structure for reduction of an insulation dimension, and further of checking flow electrification.
In accordance with this invention, there is provided a stationary induction apparatus having a winding composed of a plurality of coil pieces and a plurality of electrostatic shield insulating layers surrounding the coil pieces, wherein each of the electrostatic shield insulating layers comprises an electric insulating layer, a first conductive layer placed on an inner surface side of the electric insulating layer, and a second conductive layer placed on an outer surface side of the electric insulating layer, and one or more of the coil pieces are surrounded by one of the electrostatic shield insulating layers to construct one coil/shield combination, while the coil pieces other than the coil pieces constructing the coil/shield combination and the coil/shield combination are surrounded by the electrostatic shield insulating layer other than the one electrostatic shield insulating layer.
In a case in which a stationary induction apparatus having a winding composed of a plurality of coil pieces and a plurality of electrostatic shield insulating layers surrounding the coil pieces is constructed such that, as stated above, each of the electrostatic shield insulating layers comprises an electric insulating layer, a first conductive layer placed on an inner surface side of the electric insulating layer and a second conductive layer placed on an outer surface side of the electric insulating layer and one or more of the coil pieces are surrounded by one of the electrostatic shield insulating layers to construct one coil/shield combination while the coil pieces other than the coil pieces constructing the coil/shield combination and the coil/shield combination are surrounded by the electrostatic shield insulating layer other than the one electrostatic shield insulating layer, since the grounded capacitance of the coil pieces are shielded so that the voltage distribution at the surge voltage application effectively depends only upon the series capacitance resulting from the electrostatic shield insulating layers to make the voltage developing between the coil pieces more uniform as compared with the conventional art, the local high stresses at winding end portions are reducible to contribute to the improvement of the insulation performance with respect to the surge voltage. In addition, it is possible to eliminate an insulation weak point section without strictly forming an insulating structure in which, for example, the oil layer section forming the weak-point portion is packed closely with insulating paper or the like as in the conventional art, and further to shorten the insulation distance. Moreover, when an oil is circulated for cooling purposes, in the electrostatic shield insulating layer, the conductive layer relieves the electrified charge growing in conjunction with a flow of the cooling oil on a surface of the insulating layer to restrain the flow electrification. This can prevent distortion of the electric field by the electrified charge and can reduce a possibility of the occurrence of insulation abnormality stemming from the flow electrification.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.